1. Field of the Invention
This invention relates to the production of acrylate-based pressure-sensitive adhesives by polymerization which is initiated via exposure of monomers to ionizing radiation. The invention further relates to the production of such adhesives in a low-solvent or solventless form suitable for hot melt application.
2. Prior Art
Pressure-sensitive adhesives are materials which characteristically possess tack, that is bonding, when applied to adherents after brief contact at low pressure, develop greater bond strength with greater application pressure, yet desirably can be cleanly and easily removed from their adherents. Originally, pressure-sensitive adhesives, which were developed for self-adhering medical plasters and bandages, were based on natural rubber. Due to its high glass transition temperature, rubber does not possess pressure-sensitive adhesive properties at room temperature. Consequently, natural or synthetic rubbers must be compounded with a variety of tackifiers, such as terpenes or rosins, and often with other additives in order to function effectively as pressure-sensitive adhesives.
Pressure-sensitive adhesives produced from natural rubber dominated the field prior to World War II, a time when many pressure-sensitive adhesive products were developed. These include tapes for medical, electrical, packaging and processing applications, as well as self-adhering labeling, construction and decorative materials. Shortages of natural rubber led to the development of synthetic polymeric materials which found application in pressure-sensitive adhesives, including synthetic rubbers based on styrene-butadiene or styrene-isoprene polymers.
Additionally, there were developed a class of pressure-sensitive adhesives based on homopolymers or copolymers of acrylic acid esters. Acrylate-based pressure-sensitive adhesives offer some significant advantages over those based on natural or synthetic rubber. Due to their low glass transition temperatures, certain acrylate polymers have good tack at room temperature and can be used without the necessity of compounding with tackifying agents. Thus, acrylate-based products provide manufacturers of pressure-sensitive adhesive products with ready-to-coat pressure-sensitive adhesives, eliminating the need for in-house compounding operations.
Monomers useful in preparing acrylate-based pressure-sensitive adhesives are generally those having from 2 to 8 carbons in the ester group. Of particular importance are ethyl acrylate, n-butyl acrylate, isobutyl acrylate and 2-ethylhexyl acrylate. If desired, these monomers can be copolymerized with other ethylenically unsaturated monomers to yield satisfactory pressure-sensitive adhesives. Suitable comonomers include, for example, acrylic acid, methacrylic acid, vinyl acetate, vinyl pyrrolidone and vinyl ethers. If desired, difunctional or polyfunctional monomers, such as diacrylates, can also be copolymerized with the adhesive polymer in small amounts to impart cross-link density.
In order to be useful, a pressure-sensitive adhesive should have a balance of properties, e.g., tack, peel strength and creep resistance, best suited to the particular end use in which the adhesive is to be employed. The properties of a pressure-sensitive adhesive are primarily affected by monomer composition, molecular weight and cross-link density, all of which determine the balance of adhesivity and cohesivity in the polymer Generally, the monomer composition determines glass transition temperature and surface chemistry, both of which affect adhesion For linear polymers, higher molecular weights normally result in better cohesion. Cohesion also increases with the degree of covalent cross-linking and secondary intermolecular bonding
It is possible to produce a low molecular weight acrylate polymer which has good adhesivity but which will fail cohesively. Cohesivity can be improved by introducing cross-links into the polymer, after it has been applied to the substrate, by exposure to ultraviolet or ionizing radiation. However, high cross-link density can cause a loss of viscoelasticity, which in turn results in a loss of tack. In addition, many webbing substances have poor radiation resistance and are unable to tolerate doses required to impart good cohesivity.
In many commercial applications of pressure-sensitive adhesives, it would be preferred to use an acrylate polymer or copolymer having an intrinsic viscosity of at least about 2.5 dl/g. Unfortunately, prior art processes for producing acrylate polymers have been limited by considerations of reaction kinetics and thermodynamics to values below about 1.8 dl/g. Since various molecular weight fractions may affect adhesion and cohesion differently, molecular weight distribution, as well as average molecular weight, affects the properties of the adhesive. Ideally, a process for producing an acrylate-based polymer for a pressure-sensitive adhesive provides a means for controlling both molecular weight, i.e., intrinsic viscosity, and molecular weight distribution.
Despite the advantages which acrylate-based pressure-sensitive adhesives offer in comparison with compounded rubber, acrylates produced by the processes of the prior art also have a number of serious drawbacks.
The conventional method of preparing pressure-sensitive adhesives from acrylate monomers involves free radical addition polymerization in an organic solvent using chemical initiators. Chemical initiators commonly employed include organic peroxides, azobis compounds, persulfates and oxidation-reduction systems. Solvents include a variety of aliphatic hydrocarbons, aromatic hydrocarbons, esters, ketones and others. The product consists of a high viscosity polymer solution between 20 and 45 weight percent polymer concentration.
The polyacrylate solution is coated onto a substrate, e.g., a backing or webbing such as tape or label stock The volatile solvent is then evaporated, thereby leaving a nearly solvent-free layer of adhesive. The practical manufacture of pressure-sensitive adhesive products such as tapes, labels and others also often requires the use of prime and/or release coatings or inclusion of a release coated interlayer.
The conventional methods for solution polymerization and coating of acrylate-based pressure-sensitive adhesives possess several inherent drawbacks. Chemical initiators react to yield free radicals, which initiate the chain reaction propagation by which the addition polymerization proceeds. In all cases, the rate of free radical generation or initiation (which affects the molecular weight of the product) is highly temperature dependent, often increasing by an order of magnitude for every 10.degree. C. temperature rise. Acrylate polymerization is a very exothermal reaction, yielding approximately 20 kilocalories per mole of monomer Reaction control is therefore limited by the ability to remove heat from the system. Failure to maintain a reasonable temperature profile can result in undesirably low molecular weight, poor product performance, poor reproducibility and, in extreme instances, violent runaway reaction. Temperature control may be accomplished by heat transfer or solvent reflux and may be aided by gradual introduction of the initiator. However, the ability to provide adequate heat removal and mixing of initiator for a commercial scale reactor is limited by the bulk viscosity of the system. Viscosity is determined by the molecular weight and concentration of the polymer. Therefore, the use of conventional solution polymerization limits both the molecular weight and final solids concentration of the product, and as a result, using the methods of the prior art, high-solids acrylate adhesives are inferior to the solution polymer analogs.
The use of chemical initiators poses a further drawback in that a low molecular weight fraction consisting of unreacted catalyst and catalyst fragments invariably will be incorporated into the adhesive. These low molecular weight species are undesirable because they are free to migrate to the adhesive bond surface, where they inhibit performance. Furthermore, for biomedical applications, such as dermal application in first aid products, the initiator fragments may irritate the skin or possibly show other toxic effects.
The conventional coating of acrylate polymer solutions and subsequent drying is also problematic. The drying phase is highly energy intensive, usually the most costly and time-consuming in the manufacturing chain. Evaporation of large volumes of solvent in the workplace may also violate environmental and occupational safety regulations and create a fire and explosion hazard. Moreover, the resulting loss of solvent is expensive. While solvent recovery systems may help alleviate both material loss and environmental damage, these systems are very expensive to install and operate.
An alternative to solution-based polymers involves the use of water-borne acrylate polymer latices which are produced by emulsion polymerization. While the use of an acrylic latex as a substitute for solutions eliminates the problems associated with the evaporation of hydrocarbons, it still requires the evaporation of large volumes of water. While the drying step may not have the same environmental consequences and pollution abatement costs, the energy requirements are greatly increased, thus it remains expensive and time-consuming. Moreover, the emulsion polymerization process by which the latex is produced requires both water-soluble chemical initiators and significant amounts of surfactants. These low molecular weight species may be detrimental to product suitability and performance. Furthermore, polar monomers with appreciable solubility in water may be difficult or impossible to incorporate into the polymer.
In response to the need to reduce energy consumption and avoid the environmental and work safety problems associated with solution and latex polymers, manufacturers of acrylate-based pressure-sensitive adhesive products have, in recent years, developed methods for curing or polymerizing acrylate monomers inplace on a substrate using ultraviolet or electron beam energy. In these methods, a liquid mixture of acrylate monomers and/or prepolymers possibly with other components, is applied as a film or coating onto a substrate, such as tape or label stock. The coatings or films usually have a thickness in the range of about 0.5 .mu.m to 500 .mu.m. The coating on the film is exposed to ultraviolet radiation (if photoinitiators are present in the coating) or electron beam radiation to polymerize the monomers and/or cure the system by the introduction of cross-links. These processes must be operated at very high free radical concentrations, and radiation exposure rates are in the megarads per second range since economic and practical considerations require that the coating be polymerized and cured in a matter of a few seconds. As a result, adhesives produced in this manner have a low degree of polymerization and contain low molecular weight fractions which often affect adhesive performance adversely. Although both ultraviolet and electron beam curing may give satisfactory results for some products, high residual monomer is often a problem and the products obtainable may be limited to highly cross-linked systems.
Additionally, the products which may be manufactured by these processes are limited to webbings which have enough radiation resistance to withstand the relatively high doses required for curing. These processes may be employed only to manufacture certain pressure-sensitive products, such as tapes, by in-situ polymerization; and they are not suitable for the production of bulk adhesives which may be subsequently applied to any number of substrates regardless of radiation resistance.
U.S. Pat. Nos. 4,165,266, 3,772,063 and 3,661,618 describe exemplary systems for in-place radiation polymerization or curing of acrylate coatings to produce pressure-sensitive adhesive products.
An alternative to the application of pressure-sensitive adhesives in the form of solutions, latices or monomer compositions for in-place radiation curing is hot-melt application. This technique, which has been used extensively for the application of compounded rubber adhesives, requires that the adhesive polymer be in a solventless or low-solvent (i.e., &gt;70% solids) form. In hot-melt applications, the viscoelastic solid is heated to its softening or melting point and applied to the substrate by means such as calendering, spraying, or extrusion. Hot-melt application of adhesives is relatively simple and avoids or ameliorates the need for evaporating solvents with the attendant energy costs and environmental hazards.
As previously indicated, the production of acrylate polymers in a solventless or low-solvent form suitable for hot-melt application is difficult, if not impossible, using polymerization procedures of the prior art. Consequently, there are few commercial hot-melt, pressure-sensitive adhesives based on acrylate polymers, and these are for narrow applications, despite the fact that the art has long sought such products. While some hot-melt acrylate adhesives have been produced by stripping off solvent from conventionally produced acrylate solution polymers, these adhesives have not been widely accepted due to their poor performance properties. Moreover, this procedure does not eliminate the need for evaporating large quantities of solvent.
Hot melt acrylic adhesives have been reported which are said to show reversable thermal cross-linking upon cooling. These and other highly cross-linked hot melt adhesives may show marked reduction in viscoelasticity and thus poorer performance.
It is a primary object of the present invention to provide a process for producing acrylate-based pressure-sensitive adhesives which provide for accurate control of molecular weight and molecular weight distribution. In particular, it is an object of the invention to provide a process which allows for the production of acrylate-based pressure-sensitive adhesives having intrinsic viscosities above about 2.5 dl/g.
It is a further object of the invention to provide a process for producing acrylate-based, pressure-sensitive adhesives which are free of residual polymerization initiators, surfactants and other low molecular weight species.
It is a further object of the invention to provide a process for producing acrylate-based, pressure-sensitive adhesives in a low-solvent or solventless form suitable for hot-melt application.
These and other objects of the invention will be readily apparent from the description that follows.